[meta] Extending DOM APIs to support shadow roots tracking issue #1475

This is a meta-issue to track requests to expand existing DOM APIs to cover shadow roots and related concepts (e.g. slotted elements, composed trees).
To keep the list manageable, only the top-level proposals for each feature are included, not individual low-level issues about ongoing work.

Expand existing APIs to shadow Roots:

• MutationObserver flag to observe mutations in (open) shadow trees or shadow attachment, to facilitate polyfills of HTML syntax #1287
• Add an optional options dictionary to closest to allow jumping across shadow boundaries #1265
• Should TreeWalker be expanded to support shadow roots in support of the selection API? #1189
• Extension only API to query across shadow trees #1321
• Proposal: new API to get text content across shadowRoot #1395
• Option to allow querySelector to query shadow roots #1422

Expand existing APIs to composed trees:

• another version of Element.prototype.contains that considers slotted elements as children #1214
• Composed tree infrastructure #725
• Match querySelector for pseudo elements (::slotted()) #463 (basically querySelector() on the composed tree)
• Allow MutationObserver to observe across <slot>s #1415